I have this weird effect after I apply shade smooth
Did you select autosmooth check box?
It is right here, it should most probably solve your problem:

If it doesn't fix it here's is another possible solution:

Click on the arrow next to viewport overlay
Select 2nd option from the bottom called face orientation.
If all the faces are blue, you are good to go, if any face is red you might have to recalculate the normals.
to recalculate normals, go too edit mode, select all the face by pressing A, press Alt + N, From the list that opens select recalculate outside.